From the author of

From the author of

Good cryptographic methods assure us that we can keep our secrets from others.
That is, Alice and Bob's encrypted files remain private between them as long
as their secret key stays secret.

Modern-day cryptographers use the term confidentiality to mean that
your encrypted secrets aren't available to unauthorized users.1 Let's
review that concept briefly and examine three other necessary electronic data
assurances-authentication, integrity, and nonrepudiation-defined in Figure
7-1.

In this chapter we explain how secret key cryptography implements these assurances.
Later chapters examine how modern cryptography uses public keys more than secret
keys for this purpose. The concepts are the same, although more involved, when
public key methods are used. So we look first at the simpler case.

Confidentiality

Why you want authentication, integrity, nonrepudiation.

Suppose Alice and Bob have a West Coast real estate business. While Bob is
on the road, Alice and Bob exchange financial and love notes encrypted with
their secret key. Strong cryptography helps Alice and Bob feel assured their
confidentiality (privacy) is being maintained because only someone who has their
secret key can make sense of their shared electronic messages (see Figure
7-2).

Strong cryptography also ensures the confidentiality of encrypted files stored
on computer disks; only those with whom we've shared the secret encrypting key
can decrypt and understand the content.

But confidentiality (privacy) is not enough assurance to give
you the warm fuzzies you crave about the security of your communications
(see Figure 7-3). Even before you send or receive encrypted data to or from another
computer, you need to know that the person on the other end
of the line is the person he or she claims to be (authentication). You also
need to know that the software you downloaded hasn't been tampered with during
its journey to you (integrity). And you'd probably also like to be assured that
your stockbroker brother-in-law can't deny that he received your sell order
before the bottom dropped out of the market. Similarly, he wants the same assurance
if you deny that you instructed him to buy a falling-star dot-com (nonrepudiation).

Figure 7-2Confidentiality is like sending your secret in a safe; only the owner of
the shared secret key can decrypt the message (open the safe).

Figure 7-3Cryptography offers a way to detect masquerading impostors and ensure the
identity of the person on the other end of the line.

1. In Chapters 7 and 8 we represent confidentiality with an
image of a safe with an encrypted plaintext symbol. We're using a safe to
reinforce the concept that encrypted text ensures privacy. After Chapter 8,
confidentiality will be shown using only the encrypted plaintext symbol (without
the safe).